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Seldom indeed can a discovery in the field
of physics have given rise to such intensive research work as did
that of Röntgen in 1896,
when he proved the existence of a new form of rays which had
hitherto been unknown and which, owing to their remarkable
characteristics, have since achieved a position of the greatest
importance, not only in the field of pure physics but also in
connection with research work throughout the other
sciences.

Notwithstanding the considerable number of tests which have been
carried out since their discovery and directed toward
investigation of the true nature of X-rays, it was not until over
a decade had passed that their true nature had finally been
elucidated.

Already during the first tests it was established that not even
the strongest magnetic fields were able to alter the direction of
the rays. It was equally impossible to prove the existence of a
refraction on transfer of the rays from one medium to another. If
the X-rays were of a corpuscular nature they could not,
therefore, be carriers of an electrical charge, as is the case
with other known rays of corpuscular nature. If, therefore, we
wish to disregard matter which has no electrical charge, it is
necessary to assume that the particles, whose motion is
characteristic for the X-rays, bear two charges of opposite sign,
one of which neutralizes the other. On the other hand, from the
fact that there was no evidence of refraction of the X-rays, it
was possible to assume that, should they consist of a transverse
wave motion - as is the case with light waves - the relevant
wavelength would have to be very small, as for very small
wavelengths, according to the theory of dispersion of light, the
refractive index would approach unity.

After hurriedly discarding an hypothesis which had been expounded
initially, according to which X-rays were believed to consist of
longitudinal wave motions in ether, opinions as to their actual
nature were divided according to the above two alternatives.
Nevertheless an objective presentation could only describe them
as a type of impulse of an unknown nature.

On the basis of an hypothesis expounded as early as 1896 by
Stokes and Wiechert this impulse was believed to consist of a
disturbance which occurs in the ether when the cathode-ray
particle, i.e. a forward-rushing electron, is impeded on
colliding with molecules of matter. This disturbance or impulse
was believed to propagate in all directions at the speed of light
from the ether surrounding the electron. In each part of the
space this disturbance was maintained for a period of identical
duration to that in which the electron was impeded. This period
of time, multiplied by the speed of light, was described as the
impulse width, a quantity which, if the nature of the X-rays were
the same as that of the light rays, would coincide with the
wavelength.

According to that theory the X-ray impulse, which originates
perpendicular to the cathode-ray bundle by which it is excited,
is alleged to be completely polarized. The evidence of this type
of polarization was first produced by Barkla in 1905, but, contrary to
the theory, the polarization was not complete but only partial.
While it was possible to explain the causative factors of this
aberration the characteristics of the polarization were not
adequate to prove the existence of a transverse undulation.

Once Dorn had succeeded, in 1897, in determining the fraction of
the energy of the impeded electrons which is converted to X-rays,
W. Wien was able to calculate the impulse width which, according
to his figures, amounted to approximately 10-10 cm, or
only one hundred-thousandth of the shortest known wavelengths of
light. The short impulse width thus determined could explain the
lack of success with previous diffraction tests which had been
carried out on slits with X-rays, for even with the narrowest
slit the diffraction phenomenon, which is produced by such small
impulse widths or wavelengths, would have to lie just about at
the limits of possible observation. And it may, in actual fact,
only be said even of the most accurate of these tests conducted
by Walter and Pohl that they render diffraction highly probable.
From the research carried out by these scientists it would
meanwhile seem to follow that the upper limit for the impulse
width of X-rays lies at 4 x 10-9.

This was the situation when von Laue placed a research medium of
the highest import at the disposal of science by virtue of his
epoch-making discovery of the interference of X-rays and, at the
same time, proved that X-rays, as is the case with light rays,
consist of progressive transversal waves.

Previous research had indicated, as is mentioned in the
foregoing, that it was highly probable that, if X-rays are wave
motions of the same type as light rays, then their wavelengths
would have to be of an order of 10-9 cm. In order to
obtain clear interference phenomena of the same type as those
which are caused when light rays pass a grating it was necessary
for the distance between the grating slits to be of an order of
10-8 cm. But this is approximately the distance
between the molecules of a solid body and it was in this manner
that von Laue arrived at the idea of employing, as a diffraction
grating, a solid body with regularly-arranged molecules, e.g. a
crystal. As early as 1850 Bravais had introduced into
crystallography the assumption that the atoms composing the
various crystals are arranged in regular groups, so-called
three-dimensional lattices or space-lattices, whose constants
could be calculated with the aid of crystallographic data.

However, the theoretical basis of a space-lattice was unknown and
thus it was first necessary for von Laue to develop this theory
if else the investigation were to have a value. This he did
mainly according to the same approximations as those conventional
to the science of optics as applied to normal one-dimensional
lattices.

Von Laue left the execution of the experimental work in the hands
of W. Friedrich and P. Knipping. The apparatus which they
employed consisted of a lead box into which they admitted a thin
bundle of X-rays which they directed so as to fall upon a
precisely oriented crystal. Sensitized film was positioned both
behind and at the sides of the crystal. Already the preparatory
tests showed that the intensity maxima which had been anticipated
by von Laue became evident in the form of blackened spots on the
film positioned behind the crystal.

From the grouping shown by these intensity maxima in accordance
with the requirements of the theory, as established, for such
photograms of various crystals and from the degree of clarity
with which they have been reproduced, it follows that they are an
interference phenomenon. Absorption tests have shown that the
rays which give rise to the points of interference are actually
X-rays, and from this von Laue has deduced with a high degree of
certainty that the X-rays which cause intensity maxima on
irradiation of a crystal have the character of a wave motion.
However, the same is required also for those rays employed for
irradiation purposes, for, as he says, were they of a corpuscular
nature, coherent oscillations could only arise from those atoms
set into motion by the identical corpuscle and these atoms would
have to form together one whole agglomerate whose dimensions
would tee largest in the direction of radiation. However,
contrary to what was indicated in the experiment, this would
result in the intensity maxima consisting of irregular concentric
circles.

As a result of von Laue's discovery of the diffraction of X-rays
in crystals proof was thus established that these light waves are
of very small wavelengths. However, this discovery also resulted
in the most important discoveries in the field of
crystallography. It is now possible to determine the position of
atoms in crystals and much important knowledge has been gained in
this connection. We can anticipate further discoveries of equal
note in the future. It is thus rendered likely that experimental
research into the influence of temperature upon diffraction will
provide the solution to the question of a zero-point energy, or
will at least be of some assistance in arriving at a solution to
this problem, as the temperature factor assumes a different value
according to whether a zero-point energy exists or not. However,
the direct results of this discovery of diffraction are of no
less importance: it is now possible to subject the X-ray spectra
to direct examination, their line spectra can even be
photographed, and science has thus been enriched by a method of
research whose full implications can not yet be fully
appreciated.

If it is permissible to evaluate a human discovery according to
the fruits which it bears then there are not many discoveries
ranking on a par with that made by von Laue. If one reflects
further on the fact that only a few years have passed since his
discovery was first published it may surely be said that, when
awarding the Nobel Prize for Physics, the Royal Academy of
Sciences will presumably seldom, if ever, be in a position of
such close agreement with the letter of the Testament as on this
occasion in deciding to award the Nobel Prize for Physics for the
year 1914 to Professor Max von Laue, for his discovery of the
diffraction of X-rays in crystals.